1. Field of the Invention
The disclosure relates generally to a system and method for measuring an overlay error, and more particularly, to a method for designing two-dimensional (2-D) array overlay targets and a method and system for measuring overlay errors using the same.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Continuing improvements in semiconductor process technology have increased the accuracy required for overlay measurement. According to the ITRS (International Technology Roadmap for Semiconductor) the overlay tolerance in the semiconductor process is about 9 nm for the 45 nm semiconductor process technology to be applied in industry in the year 2010, and the corresponding metrology error budget is only about 0.8 nm. Therefore, due to diffraction limitations and proximity effects, conventional image-based overlay metrology cannot meet the accuracy requirements for the next-generation structural parameter measurement.
Diffraction-based metrology, which is different from image-based overlay metrology and is not influenced by the proximity effects, has high repeatability and reproducibility characteristics and will become an important overlay measurement technology.
An overlay measurement system using diffraction-based metrology technology is comprised of a scatterometer and a program for matching analysis, wherein the analysis technique can be a theoretical model-based method or an empirical model-based method. The theoretical model-based method utilizes electromagnetic theory to simulate a characteristic diffraction spectrum of a grating illuminated by an incident light, and then establishes a theoretical model-based comparison database. The characteristic diffraction spectrum is varied using different geometrical parameters (line widths, sidewall angles, thicknesses, overlay errors, etc.) and different optical characteristic parameters (refraction indexes, extinction coefficient properties, etc.) of the grating. In practice, the parameters such as line widths, thickness, sidewall angles and overlay errors are typically strongly correlated, and any incorrect parameter will result in incorrect overlay calculation. U.S. Pat. No. 7,283,237 discloses an optical measurement system using a theoretical model-based method to establish a comparison database. However, the designed 2-D grating is not able to easily isolate the changes induced by the error in the X direction from the error in the Y direction.
The empirical model-based method compares measured data with an empirical regression line, which approximates the data obtained and analyzed by measuring the diffraction spectra formed by a series of at least four gratings having different overlay deviation patterns. These gratings are fabricated on a wafer. The advantage of this method is that it does not require a lot of simulation for setting up a matching database, and therefore there is no strong correlation problem between the parameters. However, one disadvantage of this method is that many gratings must be fabricated and measured on a semiconductor wafer for gathering the library data, which requires a lot of time.